This invention relates to cordless digitizer systems and, more particularly, to a drive circuit for outputting a square wave drive signal of known initial polarity to a transducer coil in the cursor of a cordless, electro-magnetic digitizing system to oscillate the transducer coil and cause a magnetic field output therefrom which is detected in an associated tablet wherein the phase of a response signal induced by the magnetic field can be determined from the response signal itself comprising, first logic circuit means for outputting bursts of a square wave drive signal of known initial polarity at a first output thereof and for outputting a signal indicating the start of a burst at a second output thereof, the first output being connected to the transducer coil; and, second logic circuit means connected to the second output and to the transducer coil for additively outputting a single large current square wave signal pulse to the transducer coil of the same polarity and in phase with a first square wave pulse of the square wave drive signal when the first logic circuit means is initially outputting a burst of the square wave drive signal to the transducer coil whereby additive initial drive currents cause the transducer coil to oscillate immediately and produce a detectable first half-wave in a response signal in the tablet thereby allowing detection of the phase of the response signal from the first half-wave of the response signal of known polarity.
In a digitizer system, a cursor is moved over the surface of a tablet by a user to effect data input. In many digitizer systems, the cursor is physically connected to the tablet by a connecting cable. This allows the position determination logic associated with the tablet to know what is happening at both the cursor and tablet and makes the job of positional determination much easier. In an electro-magnetic digitizer system with a driven coil, an AC electrical signal is connected to a coil in the cursor through the cable. The signal causes the coil to emit a changing magnetic field that induces signals into the wires of a grid system in the tablet. The wires of the grid system are sequentially and alternatively scanned in the X and Y directions of an X-Y coordinate system associated with the tablet. The size of the signal in the respective wires of the X and Y grids changes with proximity to the position of the cursor (i.e. the coil). Most important, the phase of the signal changes from one side of the coil to the other since the magnetic field from the coil cuts the wires in opposite directions from one side of the coil to the other. Using this data knowledge, the positional logic can then use interpolation techniques well known in the art to find the point where the signal passes through zero (i.e. changes phase) and, thereby, the exact position of the cursor.
Elimination of the cord between the cursor and the tablet can provide many benefits to a user in the way of convenience of use. This is particularly true in large tablet systems where the cord can become long and cumbersome. Thus, more recently, cordless digitizer systems have been made available commercially. In a cordless digitizer system, there are many technical advantages to employing an electro-magnetic approach such as that described above for a cord-connected system. Because very low signal levels are employed in digitizer systems in general, they are susceptible to outside interference from various sources of radiation that can be present in the environment of use such as cathode ray tube displays, and the like. An electro-magnetic system allows the designer and builder to maximize the valid data from the signal of interest while minimizing the effects of undesired interference such as electrostatic fields.
In a cordless looping digitizer system as developed by the assignee of this application, a battery operated circuit within the cordless cursor outputs bursts of a square wave drive source voltage signal as depicted in FIG. 1 which causes an oscillating circuit response output in and from the coil as shown in FIG. 2. As indicated at 10 in FIG. 2, the initial response is so low that the detecting circuitry and logic in the tablet is unable to detect and determine the phase of the signal. As a consequence, a third, seven-wire loop has been disposed in the tablet in combination with the two X and Y grid loops from which to obtain a phase reference signal. While the extra looping does provide the required phase reference signal, it also adds to the manufacturing costs and design flexibility of the digitizer. Additionally, flexible cordless looping digitizers will not work if an aluminum shield is employed as in other digitizers and system errors can occur due to added resistance and the capacitance of the phase reference loop.
Wherefore, it is the object of this invention to provide a means of causing the coil to transmit an easily-detectable pulse which will identify the polarity of the transmitted signal which can be combined with a simple detection method to eliminate the zero.degree. or 180.degree. ambiguity that occurs in systems using a looping polarity sensitive technique.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.